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Light continued
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Refraction of light is the bending of light as it goes from one optical medium to another
Less dense to more dense: bends towards normal More dense to less dense: bends away from normal
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A medium is
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Index of Refraction
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Incident ray i r Refracted ray Glass block
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The Laws of Refraction of Light
1. The incident ray, the normal and the refracted ray all lie in the same plane where n is a constant This is called Snell’s Law
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When light travels from a rarer to a denser medium it is refracted towards the normal e.g. air to water
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when light travels from a denser to a rarer medium it is refracted away from the normal e.g. glass to air
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Experiment to Verify Snell’s Law and determine the refractive index of glass
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i 1 2 r Block of glass 3 4
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Experiment to Verify Snell’s Law and determine the refractive index of glass
Method Outline the glass block on paper Stick pins 1 and 2 in the paper in front of the block Stick pins 3 and 4 in line with the images of 1 and 2, as seen through the block
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4. Remove the pins, join the pinholes and draw the normal
5. Measure angles i and r 6. Repeat for different angles of incidence 7. Draw a graph of sin i vs. sin r 8. The slope of the graph is the refractive index of glass
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Result i r Sin i Sin r Sin i/Sin r
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Sin i Sin r
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Slope of graph = Thus the refractive index of glass is ____
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Conclusion The graph is a straight line graph through the origin, thus verifying Snell’s Law The slope of the graph gave the refractive index of glass
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Real and Apparent Depth
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A swimming pool appears to be less deep than it actually is, due to refraction at the surface of the water We can calculate the refractive index of a liquid by using n =
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Refractive index n =
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Question 1 When light passes from air into a liquid, the angle of incidence and refraction are 57˚and 30˚ respectively. Calculate the refractive index of the liquid. Answer Formula:
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Question 2 A ray of light is incident at 35˚ on (a) a glass surface and (b) a water surface. Calculate the angle of refraction given that, ng=1.52 nw=1.33 Answer Formula:
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Textbook Examples Questions 1-3 on page 31
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Total Internal Reflection
This may occur when light goes from a denser to a less dense medium As i is increased so is r Eventually r = 90˚ At this point i has reached the ‘critical angle’
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r = 90˚ i = critical angle
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Total Internal Reflection!!!
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It is reflected back into the first medium
If i is increased beyond the critical angle, the ray does not enter the second medium It is reflected back into the first medium We can also find the refractive index of a material using n = C = critical angle
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Example The critical angle of glass is 41.81˚
Find the refractive index of glass n = n = 1/0.666 n = 1.5
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Refractive index n =
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Example The refractive index of glass is 1.5
This value is for a ray of light travelling from air into glass So ang = 1.5 = Or gna = =
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Applications of Total Internal Reflection
Periscopes (using a prism) Diamonds and bicycle reflectors Optical fibres – in telecommunications and by doctors
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Prisms Prisms are considered to be better than mirrors for reflecting light. This is because they use total internal reflection. This is the reason prisms are used in periscopes and binoculars.
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Optical Fibres Total Internal Reflection has given rise to the use of optical fibres. The signal is fired at an angle greater than the critical angle so that it is reflected all the way along the fibre. The light is therefore trapped within the fibre. This technology is used to transmit information over long distances.
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Endoscopes These are used in medicine. They contain two bundles of optical fibres, one to carry light to the organ being viewed and the other to carry the image of the organ back to the operator.
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Optic Fibre Glass cladding of low refractive index
Glass core of high refractive index
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Glass of high refractive index
Glass of low refractive index N N Normal
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Remember that rays are path-reversible
AIR GLASS A B
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Mirages Mirages are caused by the refraction of light in air due to temperature variations
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SKY
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Convex lens (converging)
LENSES Made of transparent material, usually glass Convex lens (converging) Thick in centre and thin at edges
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Concave lens (diverging)
Thin in the centre and thick at the edges
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Ray diagrams for lenses
1. Ray incident parallel to principal axis is reflected out through focus 2. Ray incident through focus is reflected out parallel to axis 3. Ray incident through optic centre continues in straight line
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RAY DIAGRAMS FOR CONVEX LENSES
Optic centre
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RAY DIAGRAMS FOR CONCAVE LENSES
Optic centre
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Geometric Optics 2.03
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The image in a concave lens is always virtual, erect, and diminished
Concave lens always has negative f and v
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Lens formulae m =
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Example An object is placed 10cm away from a convex lens of focal length 12 cm. Calculate the nature, position and magnification of the image.
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f = 12 u = 10 v = ? v = -60cm
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m = m = = 6 So the image is formed 60cm from the lens It is virtual Magnification is 6
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Example 2 An object is placed 40cm from a concave lens of focal length 50cm. Find the position, nature and magnification of the image.
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u = 40cm f = -50cm v = ?? v = cm
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m = m = = 0.56 So the image is formed 22.2cm from the lens It is virtual Magnification is 0.56
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Experiment to Measure the Focal Length of a Convex Lens
Pin Plane mirror
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Experiment to Measure the Focal Length of a Convex Lens
Method A rough estimate of the focal length may first be obtained by focusing the image of a distant object on a sheet of paper Set up the apparatus as in the diagram Move the pin in and out until there is no parallax between the pin and its image Measure the distance from the pin to the centre of the lens. This is the focal length
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Result The distance from the pin to the centre of the lens was ______ Conclusion The focal length of the lens is ______
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Experiment to Measure the Focal Length of a Concave Lens
Convex lens Pin F Plane mirror
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Method Set up the apparatus as in the diagram
The focal length of the convex lens is first measured The concave lens is combined with the convex lens (of shorter focal length), and so overall it behaves as a convex lens Move the pin in and out until there is no parallax between the pin and its image Measure the distance from the pin to the centre of the lens. This is the focal length of the combination
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Results The focal length of the convex lens is 22cm
The focal length of the combination is 51cm. The focal length of the concave lens can be calculated from the formula Where F = focal length of combination f1 = focal length of convex lens f2 = focal length of concave lens
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F = 51cm f1 = 22 f2 = ??
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Conclusion The focal length of the concave lens is 38.69cm
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